1. Field of the Invention
The present invention relates to collector assemblies used for collecting spent electrons in linear beam electron devices. More particularly, the invention is directed to a multistage depressed collector and mounting structure for miniature traveling wave tubes used in elevated temperature environments, such as airborne applications.
2. Description of Related Art
Linear beam electron devices, such as traveling wave tubes, are well known in the art for generating and amplifying high frequency signals. In a linear beam device, an electron gun comprising a cathode and an anode generates a linear beam of electrons. The generally cylindrical electron beam passes through an interaction structure in which a portion of the beam energy is transferred to an electromagnetic signal within the interaction structure. After exiting from the end of the interaction structure, the spent electrons of the beam pass into a collector structure that decelerates and captures the electrons in order to recover a portion of their remaining energy. Electrodes disposed within the collector structure are used to collect the spent electrons at close to their remaining energy in order to return power to the source powering the linear beam electron device. Collector structures thereby increase the overall DC to RF conversion efficiency of traveling wave tubes and other linear beam electron devices. Unrecovered beam energy is transformed into heat within the collector. To avoid overheating of the collector, this heat must be transferred out of the collector and dissipated to the external environment via a heat sink or like device.
Collector structures generally comprise a central electrode structure supported by a core of thermally rugged electrical insulating material, such as a ceramic material. The ceramic insulating material may be housed in a metal cylinder or sleeve, which is in turn fitted within a relatively massive heat sink. The core insulates the electrode electrically from ground and provides voltage isolation between electrode stages. In addition, the insulating material conducts waste heat from the electrodes to the outer housing and heat sink. The outer housing further provides a vacuum wall for the linear beam electrode device.
Collector structures of this basic type are known as depressed dual stage and multistage designs. Electrons on a spent beam are typically distributed over a range of spectral energies. The lowest-energy electrons are collected in a first, least-depressed stage electrode of the collector, and higher energy electrons progress to a second or subsequent stage electrode. The power density of collected electrodes may be particularly high in the second stage electrode. High power densities, in turn, can create thermal stresses in the collector that may cause collector failure due to melting or cracking of the insulators that support the second stage electrode. Thermal stresses are particularly high for traveling wave tubes that operate in high temperature environments, such as greater than about 200xc2x0 C.
In prior art assemblies, thermal stresses often arise from differences in rates of thermal expansion between the ceramic core and heat sink. In particular, the metal heat sink expands at a higher rate than the ceramic core, reducing heat transfer between the heat sink and the core. The reduced heat transfer to the heat sink increases the operating temperature of the core. This, in turn, can cause cracking of the ceramic core caused by expansion of the inner metallic electrode, or even melting of an electrode. In theory, these problems may be reduced by constructing the entire collector for assembly at the anticipated operating temperature. However, assembling the entire collector in an elevated temperature environment is not practical. Also, collector components, even if assembled at operating temperature, are still subject to cyclical stresses from excursions below or above the anticipated operating range.
A different type of collector assembly is disclosed in U.S. Pat. No. 6,320,315. A sleeve is comprised of a material having a rate of thermal expansion different from that of the heat sink and is disposed in close contact with the heat sink when the collector is at an elevated operational temperature. A slight gap is defined between the collector core and the sleeve when the collector is at an ambient temperature, and the collector core is in close contact with the sleeve when the collector is at the operational temperature. The electrode assembly is of a conventional design. The heat sink further comprises either copper or aluminum, the sleeve is comprised of molybdenum, and the collector core is comprised of a ceramic material. To assemble the collector structure, the heat sink is heated to a temperature above the operational temperature, and the sleeve is inserted. The ceramic core is then inserted into the sleeve at an ambient temperature. Although this design provides useful benefits, further cost reductions and performance improvements are still desirable.
It is therefore desired to provide a collector structure having a ceramic collector core that permits sustained operation at high temperatures and high power densities, such as encountered in miniature traveling wave tubes. More particularly, a collector assembly that provides efficient heat transfer from the collector core at elevated temperatures is desired while reducing stresses on collector components caused by thermal cycling. It is further desired to avoid concentrated power densities in the second stage electrode. In addition, the collector assembly should be relatively inexpensive to construct.
The present invention provides a novel collector structure for a linear beam device that overcomes the limitations of the prior art using a new and innovative design. The collector structure comprises a heat sink having a vacuum cavity, a segmented ceramic insulator within the cavity, and an electrode assembly within the ceramic insulator.
The collector includes a ceramic insulator (core) that is segmented into two or more (such as three) preferably axisymmetric sectors that fit together to surround the collector electrodes. A notched butt joint is preferably used at the interfaces between the ceramic pieces to maintain electrical isolation of the electrode and to reduce concentrated electric fields in the ceramic throughout the operating temperature range. The notches provide reliable high voltage standoff. The individual segments of the ceramic insulator are not attached to one another. No sleeve is needed between the ceramic and. the heat sink, and the ceramic insulator is preferably inserted directly into a cavity of the heat sink.
In an embodiment of the invention, molybdenum is used for the second stage collector electrode. A probeless electrode shape with a deep rear taper is preferably used to reduce power densities in the collector and provide better power dissipation. The first stage electrode may be comprised of copper and be conventionally shaped.
The heat sink may be comprised of a molybdenum material, instead of conventionally-used copper or aluminum. Preferably, the heat sink also provides the vacuum wall for the collector. Molybdenum is preferred because it is a refractory material with a low coefficient of thermal expansion, good thermal conductivity, and low vapor pressure at elevated temperatures. An outside surface of the heat sink may be shaped to conform to a round shaped air cooled surface, or such as an outer surface of a final assembly, thereby eliminating a thermal interface and improving heat exchange to the external environment.
In an alternative, lower-cost embodiment, copper may be used for all of the electrode stages, and copper is also used for the heat sink material. The heat sink, insulator, and electrode are sized such that the insulator and the electrode are compressed by the heat sink at ambient temperature and throughout the operating range of the collector. The remaining aspects of the collector may remain substantially the same as for the molybdenum collector and heat sink. Advantageously, copper is less expensive than molybdenum materials, although not as ideally suited for high-temperature, high power density operation.
The present invention provides several advantages. The segmentation of the ceramic insulator relieves thermal stresses on the ceramic while maintaining good heat conduction to the heat sink over a wide range of operating temperatures. In an embodiment of the invention, the electrode and the heat sink expand and contract at approximately the same rate, and so the pressure exerted on the ceramic insulator between these components remains relatively constant. In the alternative, the electrode has a different coefficient of expansion, preferably a higher coefficient of expansion, than the heat sink. For example, a copper electrode may be used with a molybdenum heat sink. In such embodiments, the compression on the ceramic insulator will increase with temperature, advantageously improving thermal contact between the electrode and the heat sink as the collector heats up.
The ceramic is preferably sized to be in contact with both the heat sink and the electrode at ambient temperature and throughout the desired operating range. Annular gaps between the ceramic insulator and the heat sink or between the insulator and the electrode may cause undesirable electric field concentration and less than optimal heat conduction, and should therefore be avoided.
The ceramic insulator typically has a different coefficient of expansion than metals, including copper and molybdenum materials. In conventional collector designs, this mismatch of expansion rates would cause thermally-induced mechanical stresses and changes in heat transfer characteristics of the collector assembly over the operating temperature range. In the present invention, the free-floating (i.e., unbrazed), segmented ceramic insulator is compressed between the expanding electrode and the heat sink as the temperature increases. The ceramic segments are subjected mainly to compressive stresses, for which ceramic materials are typically exceeding strong. Little or no tensile stress can occur because the insulator is segmented. Meanwhile, good thermal contact is maintained between the electrode and the heat sink throughout the operating range. The braze-less design also allows for a wider selection of ceramic materials.
A further benefit of the invention is that the collector assemblies are relatively easy to assemble. It is not required to heat the components of the present invention in order to assemble them. The collector electrode stages may be fit together in interlocking relationship with the sections of the ceramic insulator. The assembled electrode and insulator may then be inserted together into the heat sink at ambient temperature. The assembly may then be held in place by seal flanges which may be brazed to the heat sink at the front and rear of the collector. The rear seal flange includes an end cap. Because the heat sink provides the vacuum wall, only a single braze operation is needed during assembly, to attach the seal flanges to the heat sink. Unlike prior art designs, the electrodes need not be brazed to the ceramic core.
A more complete understanding of the collector assembly will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly.